For printing devices like electrophotographic printers, color calibration, which is adjustment of colors output from a printing device, is important to maintain constant print quality. As a method of keeping the print quality, there is known a method of using a tone reproduction curve or a one-dimensional LUT (look-up table) prepared for each of the primary colors of C (cyan), M (magenta), Y (yellow) and K (black) to carry out a tone adjustment for each primary color. Such color calibration using one-dimensional LUTs can be performed with a reduced calculation load and can be processed in short time, but hardly handles a change in color balance of the primary colors. From the point of view of maintaining constant color balance of the primary colors, various types of color calibration using a multidimensional LUT like a three-dimensional or a four-dimensional LUT have been proposed, for example, in JP-A No. 2007-089031 which corresponds to US2007/0070371A1.
In general, a printing device is calibrated mostly by using one-dimensional LUTs in order to solve a fluctuation of color reproducibility which can be made in the course of several days in normal operation, and is calibrated mostly by using a multidimensional LUT at the initial adjustment in the manufacturing process of the printing device or at a time when the operation environment of the printing device has been changed significantly, for example, at a use of different material lot or at a change of season. The color calibration using a multidimensional LUT includes a process of a feedback (abbreviated to FB) adjustment to improve the color accuracy of the color calibration, by printing a color chart corrected with a color correction LUT, measuring color of the color chart printed, and updating the color correction LUT, as disclosed in, for example, JP-A No. 2012-023627 which corresponds to US2012/0013923A1.
The technology disclosed in JP-A No. 2012-023627 employs a feedback adjustment of a device-link profile by using relative white color. In concrete terms, in the technology, the mean of actual measurement errors (color differences) between two kinds of color values is calculated. First is color values which are in a device-independent color space and indicate colors of color reproduction characteristics of a target device, corrected on the basis of paper white. Second is color values which are in a device-independent color space and are obtained by color measurement of an output of a color image given with a printer. If the mean of the actual measurement errors has the value of three or more, creation of a device-link profile is repeated. On the other hand, if the value of the mean is less than three, creation of a device-link profile is ended.
In other words, the technology disclosed in JP-A No. 2012-023627 removes paper white, which is unadjustable, from colors, and performs color evaluation. On the other hand, in well-known printing standards, including Japan Color, FOGRA and IDEAlliance, color evaluation is performed on the basis of the absolute white color. Those standards support different color values of paper white and a different color gamut from each other. Therefore, the method of the feedback adjustment of a device-link profile disclosed in JP-A No. 2012-023627 can hardly judge whether to perform the feedback adjustment properly according to paper type.